1. Field of the Invention
This invention relates to an light-emitting module.
2. Background of the Invention
The WDM (Wavelength Divisional Multiplexing) system at 1.55 um wavelength band defines the adjacent wavelength interval to be 0.8 nm. This regulation means that the absolute accuracy more than ±0.1 nm is required for a specific channel. A semiconductor laser with a feedback control, such as DFB laser (Distributed Feedback Laser) and DBR laser (Distributed Bragg Reflector) is applicable for the signal source of the WDM system.
These feedback lasers show a sharp oscillation spectrum with a typical oscillation width below 50 GHz. However, the Bragg grating formed within a laser chip solely determines the oscillation wavelength. To get the desired oscillation wavelength is quite difficult due to the uncertainty of the process parameter.
It is known that the oscillation wavelength of individual lasers can be adjusted by the feedback control after the completion of the production. 1) Dividing the output light from the optical module, 2) monitoring the divided light with a spectrum analyzer, and 3) adjusting the temperature of the laser and the injection current to the laser controls the oscillation wavelength. However, this technique uses the optical spectrum analyzer and is quite impossible to apply to the WDM system, which requires the multi channel optical signal source, hence a plurality set of equipment.
Another example is disclosed in U.S. Pat. No. 5,825,792, in which the parallel plates Etalon is used for the controlling of the oscillation wavelength. The method of the '792 controls the oscillation wavelength with high accuracy, but to control the optical output power requires a complicated circuit.